1. Field of the Invention
This invention is directed toward color television receivers and in particular toward those color television receivers including automatic hue control whereby flesh-tone response is most accurately controlled without distorting other reproduced colors.
2. Description of the Prior Art
Generally, present-day color television receivers include circuitry for selective modification of hue signals by a viewer to effect a desired flesh-tone response. Factually, a relatively constant flesh-tone response is provided automatically in most present-day television receivers without any undue attention or adjustment by a viewer.
Specifically, many forms of hue compensation circuitry for flesh-tone enhancement include apparatus for phase-shifting a chrominance signal applied to the color demodulator stages. For example, U.S. Pat. No. 3,525,802, entitled "Hue Expander Circuit", issued Aug. 25, 1970, in the name of P. J. Whiteneir, Jr., provides apparatus for automatically shifting signals in the red and yellow sections of the chrominance diagram in a manner which provides a hue representative of flesh tone.
In another known form of hue compensation apparatus, chrominance signals are shifted by a phase-shift network of passive components series-connected to a chrominance signal source. Such apparatus appears on pages 104 and 105 of an article entitled, "Solid State Controls Head New Color TV Lineup", in the June 1970 edition of Electronics. Thus, relatively inexpensive passive components are substituted for relatively expensive active components in an effort to provide the desired enhanced flesh-tone control.
In still another form of hue compensation apparatus, U.S. Pat. No. 3,654,384, entitled "Apparatus for Modifying Electrical Signals", issued Apr. 4, 1972 in the name of John M. Kresock, suggests hue modification circuitry wherein the phase angles of the reference signal applied to the demodulators as well as the magnitudes thereof are altered to effect an improved flesh-tone reproduction. Thus, a shift in phase separation of the demodulation axes as well as a change in the magnitude of the reference signals applied to the two demodulators provides a desired shift in flesh tone.
Although the above-mentioned systems have been and still are widely accepted in present-day color television manufacture, it has been found that each leaves something to be desired. For example, systems which include active components are relatively expensive and appear to be more subject to catastrophic failure than circuits with passive components.
Also, circuitry employing passive components wherein the desired flesh-tone region appears to be enhanced at reduced cost of components and an increased reliability, has been found somewhat undesirable in image reproductive capabilities. More specifically, it has been found that circuitry wherein the R-Y and B-Y reference axes are shifted to an angle greater than 90.degree. tend to provide an image response wherein green signals appear blue rather than green due to the shift in the output of the B-Y demodulator from a negative to a positive value. Obviously, such an undesired shift in color response is deleterious to truly authentic and desired image reproduction capabilities. An explanation of the shift of green signals to appear blue is provided below with respect to FIG. 1.
Referring to the prior art chromaticity drawing of FIG. 1, a first reference or R-Y signal available from the reference oscillator signal source normally is depicted as lagging a color burst signal by a phase angle of 90.degree.. Also, a second reference signal or B-Y signal or is normally depicted as lagging the color burst signal by a phase angle of 180.degree.. Thus, normal operation provides a first reference signal R-Y lagging the burst signal by 90.degree., and a second reference signal B-Y lagging the burst signal by 180.degree. and lagging the first reference signal by 90.degree..
To illustrate the effect of the phase-shift network normally employed in flesh-tone or hue modification circuitry as described above, it may be assumed that the normal phase-shift network is altered. Thereupon, the R-Y and B-Y reference signals are shifted to a positional location, indicated as (R-Y)' and (B-Y)', having an increased phase angle therebetween which is preferably in the range of about 130.degree. as compared with the previous 90.degree..
Assuming a vector "A" representative of a flesh-tone on the phase diagram of FIG. 1, there would be provided a positive-going R-Y vector component represented as "a", and a negative-going B-Y vector component represented as "b". Moreover, alteration of the phase shift network to enhance flesh tones causes a shift in the phase angle intermediate the R-Y and B-Y vector components from the above-mentioned 90.degree. value to an angle of about 130.degree.. Thereupon, the new R-Y vector component a' remains positive-going, while the new B-Y vector component b' remains negative-going. Thus, the polarity of the R-Y and B-Y vectors remains unchanged and the desired flesh-tone feature is attained.
However, assuming a vector "B" represents the color green on the phase diagram, there would be a negative-going R-Y vector component represented as "c" and a negative-going B-Y vector component represented as "d". Upon alteration of the phase-shift network to enhance flesh tones, the phase angle shifts from 90.degree. to about 130.degree., whereupon the negative-going R-Y vector component "c" remains as a negative-going component c', while the negative-going B-Y component "d" shifts to a positive-going component d'. Thus, the previously-mentioned vector B representative of the color green now undesirably appears as a bluish color due to the shift in the B-Y vector component "d" from a negative-going value to a positive-going value d'. As a result, it can be seen that, while it is desirable to shift R-Y and B-Y reference signals to increase the phase angle therebetween to 130.degree. for flesh-tone colors represented by vector A, it is undesirable to do so for other colors such as green, represented by vector B.
In order to prevent the distortion of green colors upon a television receiver display, it has been suggested by U.S. Pat. No. 3,882,534 by Gopal Krishna Srivastava, that the R-Y reference signal, according to which of the red signals are reproduced, be used to determine selectively when the phase shift between the R-Y and B-Y reference signals is effected. Thus, the normal phase shift of 90.degree. between the R-Y and B-Y signals is maintained during that portion of the phase spectrum in which the green G-Y signal occurs, such that substantially no distortion occurs in its reproduction. Referring to FIG. 1, the operation of the automatic tint control of the above-noted patent is seen as effecting the desired phase shift between the R-Y and B-Y difference signals for positive values of the difference signal R-Y whereby the reproduction of the flesh-tone signal "A" is improved. However, for negative values of R-Y, the phase-shifting circuit is turned off, whereby the green signal "B" is not affected and therefore, the display thereof is not distorted. In particular, the R-Y reference signal as obtained from its color demodulator, is compared with respect to a quiescent voltage provided by a DC restorer circuit, corresponding to the outputs of the R-Y demodulator when no color signal is produced. The DC restorer as described in the noted U.S. Pat. No. 3,882,534 involves a fairly complex circuit that is responsive to the horizontal deflection circuit. A comparison and switching circuit also are involved whereby the DC quiescent voltage, as established by restorer circuit, is compared with the R-Y signal. As a result, when the R-Y signal is positive, as indicated by the output of the comparator circuit, the switch circuit is rendered conductive, whereby a phase-shifting circuit is rendered active to impart the desired phase shift between the R-Y and B-Y signals before they are imposed upon their respective demodulators. As mentioned above, one problem with the circuit as described in the noted patent is that its circuitry tends to be complex and therefore expensive. Secondly, the reference phase-shifting circuit is energized in response to the R-Y signal only to effect the desired enhancement of the flesh-tone signals. However, the yellow-green signals which are approximately in the phase vicinity of the B-Y axis, are not flesh-tone enhanced, since in the yellow-green signals, the R-Y signal which energizes the switching circuit is either not present or of a significantly lower level such that it will not energize the switching circuit.